The present invention relates generally to a solid-state laser system and, in particular, to a solid-state laser that is passively cooled and thermally controlled by heat sink bodies containing phase change material.
Solid-state laser systems are characterized in that they have a solid-state laser gain medium which converts energy from an optical pump source to a coherent output laser beam. The pump source can be one of many available energy-producing systems such as flash lamps or semiconductor laser diodes. The energy produced by the pump source is incident upon the laser medium and absorbed by the laser medium.
The absorbed energy in the laser medium causes the atoms in the laser medium to be excited and placed in a higher energy state. Once at this higher state, the laser medium releases its own energy which is placed into an oscillating state by the use of a laser resonator. The laser resonator includes at least two reflective surfaces located on either side of the laser medium. The laser resonator can be designed to continuously release a laser beam from the system. Alternatively, the resonator can be designed such that when the energy oscillating through the laser medium reaches a predetermined level, it is released from the system as a high-power, short-duration laser beam. The emitted light produced from the solid-state laser system is generally coherent and exits the system in a predefined area.
In many systems, the laser medium is Neodymium-doped, Yttrium-Aluminum Garnet (Nd:YAG). A laser medium made from Nd:YAG absorbs optical energy most readily when the energy is at a wavelength of approximately 808 nanometers (nm). Thus, the source to pump the Nd:YAG laser medium should be emitting light energy at approximately 808 nm. Gallium arsenide semiconductor laser diodes can be manufactured with dopants (e.g. aluminum) that will cause the emitted light to be in a variety of wavelengths, including 808 nm. Thus, the semiconductor laser diodes, which are lasers by themselves, act as the pump source for the laser medium.
The conversion of optical energy into coherent optical radiation is accompanied by the generation of heat which must be removed from the device. Cooling of the laser medium reduces the build-up of temperature gradients and, thereby, the strain and stress in the laser medium and also avoids the likelihood of laser medium fracture due to high thermo-elastic stress. Also, variation of the refractive index and its associated optical distortion can be largely controlled or avoided by effective cooling. The result is improved beam quality and/or increased average output power.
Diode array performance is also strongly dependent on temperature. Not only is the output power a function of temperature, but the wavelength of the emitted energy that is to be absorbed by the laser medium is also a function of diode temperature. To maintain desired array performance and to prevent the diode array from being destroyed by overheating, cooling of the area surrounding the array is also important.
Other laser assembly components, some having low damage thresholds, also require close temperature control. For example, beam dumps, that absorb and dissipate incident laser energy to ensure that incident laser energy will not emerge to interfere with wanted parts of the beam, produce heat. Nonlinear crystal assemblies for the conversion of wavelengths in a laser system utilize temperature control systems for the precise control of these temperature-sensitive crystals. Careful attention is also given to the optimal transfer of heat from acousto-optic Q-switches.
It has been an objective for laser manufacturers to develop high-power, solid-state systems. As the output power in these system increases, the waste heat increases which puts more demands on cooling systems and necessitates larger volumes in which to provide adequate cooling. Hence, the efficient and effective removal of waste heat from diode arrays, the laser medium, and other heat-generating components is an important factor in developing compact, high-powered laser systems.
Known laser systems utilize active cooling. Active cooling systems may use thermoelectric coolers, or fluid systems having mechanical pumps and coolant carrying tubing operated at pressure. However, active cooling systems consume additional power to control the temperature of the laser and require additional space in the laser system. Furthermore, active cooling requires feedback control systems to adjust the amount of cooling that is necessary to maintain the laser components at the appropriate temperature.
The present invention is a passively cooled, diode-pumped solid-state laser system producing a high-power laser beam. The system includes at least one diode array producing optical energy that is absorbed by a solid-state laser medium. The solid-state laser medium has an outer surface into which optical energy from the diode array is emitted.
The laser system further includes a pair of opposing reflective surfaces substantially optically aligned along a central axis of the laser medium and positioned with the laser medium therebetween. One of the opposing reflective surfaces is an output coupling mirror for reflecting a portion of energy produced by the laser medium to provide laser resonation and also for transmitting the high-power laser beam.
To provide the passive cooling of the laser medium, a laser medium heat sink assembly contains a substantially solid form of phase change material in thermal communication with the laser medium. The solid form of the phase change material changes to a liquid form of the phase change material in response to heat from the laser medium being transferred to the laser medium heat sink assembly.
To absorb the heat from the diode array, a diode array heat sink assembly contains a substantially solid form of phase change material in thermal communication with the diode array. The solid form of the phase change material changes to a liquid form of the phase change material in response to heat from the diode array being transferred to the diode array heat sink assembly.
While the laser system cannot be operated endlessly with only passive cooling, passive cooling can provide the necessary cooling for a laser system for several minutes. Such a system can be useful in many applications such as the terminal guidance system for a missile. Advantages to be gained from passive cooling include more compact, portable, lighter, and vibration free laser systems. Additionally, a laser system with more effective passive cooling can accommodate the increased heat transfer associated with a more powerful laser.
Furthermore, employing a phase change material in combination with the heat exchanger having a working medium flowing therethrough provides temperature control of laser components in addition to heat absorption properties. Thermal control is provided by the latent heat associated with the phase change material. A material in its solid phase will continue to absorb energy and remain at a constant temperature (its melting point) until a specified amount of energy is absorbed completing the transition from solid to liquid phase. Furthermore, an interface in intimate contact with the phase change material proceeding through this transition will be held at approximately a constant temperature until the transition from solid to liquid is complete.
To provide for more continuous operation of the laser system using a phase change material, the heat sink assembly containing phase change material is placed in thermal communication with a heat exchanger containing working fluid. The liquid form of the phase change material changes to a solid form in response to heat being transferred from the heat sink assembly to the heat exchanger. Also, the heat exchanger can be operated in reverse (i.e. transfer heat from the working fluid, or a heater, to the phase change material) to liquefy the phase change material and, thereby, maintain temperature-sensitive components at optimal operating temperatures.
The heat sink assembly containing phase change material provides a thermal buffer for laser components when the ultimate heat sink, such as the ambient air, is subject to temperature fluctuations. The thermal buffer is associated with the latent heat of fusion of the phase change material as it undergoes a phase change. The temperature of the laser component generally remains constant as the energy associated with changes in ambient temperature is absorbed in the phase change material before it is transferred to the laser component. The thermal control provided by the phase change material alleviates the need for an electronic thermal-control loop.
Additional thermal control qualities are provided by another embodiment in which a heat sink assembly containing phase change material is placed in thermal communication with a thermoelectric cooler. With the thermoelectric cooler disposed between the temperature-sensitive component and the heat sink heat is transferred from the component, across the thermoelectric cooler, and into the heat sink. With the heat sink assembly disposed between the temperature-sensitive component and the thermoelectric-cooler, the phase change material is maintained in its melt phase as heat is removed from the phase change material by the thermoelectric cooler. Also, the thermoelectric cooler can be operated to discharge heat into the heat sink assembly if it is desired to raise the temperature of any system component.
In another embodiment, the heat sink contains more than one type of phase change material, each having a different melting temperature. In this embodiment, the thermal gradient can be tailored, for example, by placing phase change material with a greater melting temperature in cavities closer to the temperature-sensitive component relative to cavities filled with a phase change material having a lower melting temperature.